This application claims the benefit of Korean Application No. 2001-43785 filed Jul. 20, 2001, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a spherical aberration compensator which compensates for spherical aberration resulting from a thickness deviation of a recording medium and an optical pickup using the spherical aberration compensator.
2. Description of the Related Art
Recording/reproduction density of a recording medium increases as a size of a light spot focused on the recording medium by an optical pickup becomes smaller. The size of the light spot is proportional to a wavelength (xcex) of light used by the optical pickup and is inversely proportional to the numerical aperture (NA) of an objective lens. Therefore, to implement a high-density recording medium, there is a need for an optical pickup with a short wavelength light source, such as a blue semiconductor laser, and an objective lens having a larger NA. Recently, there is an increasing interest in a format for increasing recording capacity up to 22.5 GB with a 0.85-NA objective lens and for reducing a thickness of a recording medium to 0.1 mm so as to prevent degradation of performance caused by tilting of the recording medium. Here, the thickness of the recording medium refers to a distance from a light receiving surface of the recording medium to an information recording surface.
As is apparent from equation (1) below, spherical aberration W40d is proportional to the fourth power of the NA of an objective lens, and to a deviation of the thickness of a recording medium xcex94d. For this reason, if an objective lens with an NA of about 0.85 is used, the recording medium must have a uniform thickness with a deviation less than xc2x13 xcexcm. However, it is very difficult to manufacture the recording medium within the above thickness deviation range.                               W                      40            ⁢            d                          =                                                            n                2                            -              1                                      8              ⁢                              n                3                                              ⁢                                    (              NA              )                        4                    ⁢          Δ          ⁢                      xe2x80x83                    ⁢          d                                    (        1        )            
FIG. 1 is a graph showing a relationship between the thickness deviation of a recording medium and wavefront aberration (optical phase difference (OPD)) caused by the thickness deviation where a 400-nm light source and an objective lens having an NA of 0.85 are used. As shown in FIG. 1, the wavefront aberration increases proportionally to the thickness deviation. Thus, if an objective lens having an NA as large as 0.85 is used, there is a need to compensate for the spherical aberration caused by the thickness deviation of the recording medium.
Japanese Patent Laid-open Publication No. 12-57616 discloses an optical pickup for detecting a thickness deviation of a recording medium. Japanese Patent Laid-open Publication No. 12-30281 describes a technique of compensating for wavefront aberration occurring where a recording medium is tilted with respect to the optical axis, using a liquid crystal compensator.
FIG. 2 shows a principle of compensating for spherical aberration resulting from thickness deviation of a recording medium 10 in a conventional optical pickup. Referring to FIG. 2, an objective lens assembly 20 (hereinafter, simply xe2x80x9cobjective lensxe2x80x9d) is designed with a group of three lens elements 21, 22, and 23 for a larger NA. The three lens elements 21, 22, and 23 are accommodated in a single bobbin 24 and are aligned on the same optical axis Xxe2x80x94X. A phase difference compensator 30 for compensating for spherical aberration is disposed on the optical axis Xxe2x80x94X on a side of the objective lens 20 opposite the recording medium 10.
In FIG. 2, a curve A drawn near the objective lens 20 shows a wavefront due to spherical aberration resulting from manufacturing errors in the recording medium 10 and/or the lens elements 21, 22, and 23. This type of spherical aberration is referred to as xe2x80x9cspherical aberration from an objective lensxe2x80x9d. A curve B drawn to overlap the phase difference corrector 30 shows the wavefront after spherical aberration compensation by the phase difference compensator 30. Another curve A1 drawn near the objective lens 20 shows the wavefront where the optical axis of the objective lens 20 is radially displaced (or shifted) from the optical axis of the phase difference compensator 30.
The phase difference compensator 30 generates an inverse spherical aberration that offsets spherical aberration from the objective lens due to manufacturing errors in the recording medium 10 and/or lens elements 21, 22, and 23. The phase difference compensator 30 uses a liquid crystal as a medium to adjust the degree of phase delay and differentially delays the phase of light by locally working the liquid crystal medium, to generate the inverse spherical aberration. Localized phase delay by the driving of liquid crystals is disclosed in Japanese Patent Laid-open Publication No. 12-30281 which is incorporated herein by reference.
The phase difference compensator 30 is operated by a separate driving circuit 40, and the driving circuit 40 operates according to a recording medium thickness variation signal, which is dynamically detected. The thickness deviation of the recording medium may be calculated from a focus error signal detected by an astigmatic lens disposed on an optical path. As an example, a technique disclosed in Japanese Laid-open Patent Publication No. 12-57616 may be applied to calculate the thickness deviation.
FIG. 3 is a graph showing a relationship between spherical aberration from an objective lens and inverse spherical aberration offsetting the spherical aberration from the objective lens. FIG. 3 shows theoretical data for a case where the objective lens 20 and the phase difference compensator 30 are coaxially positioned. Accordingly, the aberration B produced by the phase difference compensator 30 has the same magnitude as the corresponding aberration A of the objective lens 20 but has an opposite sign. Therefore, the aberration of the objective lens 20 is completely offset by the inverse aberration of the phase difference compensator 30, thereby eliminating the spherical aberration, as indicated by C in FIG. 3.
In an optical pickup, the relative position of the objective lens 20 with respect to the phase difference compensator 30 varies while the objective lens 20 is driven by an actuator (not shown) to trace a track of the recording medium 10. As a result of the relative displacement of the objective lens 20 from the phase difference compensator 30, wavefront mismatching occurs, as shown in FIG. 4. In particular, where an optical axis X2xe2x80x94X2 of the objective lens 20 is shifted from the optical axis Xxe2x80x94X of the phase difference compensator 30 by a predetermined distance xcex94L, the aberration is abnormally compensated due to the wavefront matching and wavefront degradation results rather than aberration compensation. As shown in FIG. 4, as the optical axis X2xe2x80x94X2 of the objective lens 20 is separated from the optical axis Xxe2x80x94X of the phase difference compensator 30, the spherical aberration is incompletely compensated as indicated by a sum of the curves A1 and B1 which yields the curve C1 as shown in FIG. 4, causing a more serious spherical aberration at the periphery of the objective lens 20. The curves A1 and B1 represent aberration of the objective lens 20 and inverse aberration produced by the phase difference compensator 30, respectively. The worsening of the spherical aberration due to the relative displacement between the optical axes of the objective lens and the phase difference compensator is a problem of conventional compensators designed to compensate only for thickness deviation of the recording medium, without considering a shifting of the objective lens.
To solve the above and other problems, it is an object of the present invention to provide a spherical aberration compensator which effectively compensates for spherical aberration caused by relative displacement between an optical axis of an objective lens and an optical axis of the spherical aberration compensator, and an optical pickup using the spherical aberration compensator.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects of the present invention, there is provided a spherical aberration compensator for compensating for spherical aberration occurring in an optical pickup, the spherical aberration compensator comprising: a central compensation portion coaxially disposed with an objective lens and which provides a phase distribution that is effective for compensating for spherical aberration resulting from thickness deviation of a recording medium; and a peripheral compensation portion which surrounds the central compensation portion and provides a phase distribution that is effective for reducing worsening of the spherical aberration caused by axial displacement of the objective lens from the central compensation portion.
To achieve the above and other objects of the present invention, there is also provided an optical pickup comprising: an objective lens disposed facing a recording medium; a light source which emits light onto the recording medium through the objective lens; and a spherical aberration compensator disposed between the light source and the objective lens, wherein the spherical aberration compensator comprises: a central compensation portion coaxially disposed with the objective lens and which provides a phase distribution that is effective for compensating for spherical aberration resulting from thickness deviation of a recording medium, and a peripheral compensation portion surrounding the central compensation portion and which provides a phase distribution that is effective for reducing worsening of the spherical aberration caused by axial displacement of the objective lens from the central compensation portion.
In the spherical aberration compensator and the optical pickup according to the present invention, the phase distribution of light transmitted through the peripheral compensation portion may be discontinuous from the phase distribution of light within the central compensation portion so that the central and peripheral compensation portions separately delay the phase of light in response to different compensation requirements.
The phase distribution of light transmitted through the peripheral compensation portion may be flat. In other words, an amount of phase delay by the peripheral compensation portion is equal throughout the peripheral compensation portion. Preferably an amount of phase delay introduced by the peripheral compensation portion is substantially coincident with an amount of phase delay introduced by the central compensation portion at the optical axis of the central compensating portion.
The spherical aberration compensator may comprise liquid crystals and a plurality of electrodes which locally drive the liquid crystals, to compensate for the spherical aberration by phase delay.